U.S. Pat. No. 4,207,251 discloses the catalyzed gas phase production of C.sub.1 -C.sub.24 organo-isocyanates by oxidative dehydrogenation of the corresponding N-monosubstituted formamides. The reactors disclosed are of stainless-steel construction. Although satisfactory yields of the higher isocyanates can be obtained according to the process of the above patent using stainless-steel reactors, poorer yields are obtained when preparing methyl isocyanate.